The present invention relates generally to optical fiber manufacturing and, more particularly, to a method for forming a high-density, multicore phase-locked fiber laser array.
There are many applications for high power lasers in areas such as marking, drilling, cutting, welding, ablation, sensing, imaging, medical/dental surgery, as well as military applications. For example, clad-pumped fiber lasers use rare earth ions (e.g., Ytterbium (Yb)) as the lasing medium, operating at wavelengths below 1.5 microns (μm). Additionally, using Erbium (Er) and Holmium (Ho) ions (which have eye-safe wavelengths above 1.5 μm), lasing occurs between a first excited state and the ground state. As such, lasing cannot be sustained unless the cavity loss due to the ground state absorption at the lasing wavelength is offset by co-doping the system(s) with either Ytterbium to form Yb—Er or Thulium to form Tm—Ho.
Attempts to increase clad-pumped fiber laser power have included the use of multiple cores. However, the output power of previously formed multiple core lasers (or large mode area LMA fiber lasers in particular) in the kilowatt range is usually either incoherent or distributed in multiple beam spots (commonly referred to as “higher-order modes”) as a result. Another problem with clad-pumped fiber lasers is that the pump power is usually injected into the inner cladding of a double-clad fiber laser through two end facets. Furthermore, the same energy eventually to be absorbed by the core(s) (so as to pump the lasing medium) is also traveling a great distance within the cladding, which can also absorb a significant amount of the energy as the length of the fiber becomes too long.
Therefore, although increasing the length of the laser fiber will increase the exposure of the core to the energy to be absorbed thereby, the increased length also increases the amount of energy absorbed in the cladding, by some efficiency ratio. Although lasers formed in coils of lengths in the kilometer regime have been known, they are not very efficient. To mitigate this problem, side-pumping through multiple ports along a long length of a clad-pumped fiber laser has been introduced.
Furthermore, earlier approaches to fabricating multicore fiber laser arrays have employed techniques such as precision-machining a glass clad rare-earth doped rod into a hexagonal shape and then cutting the rod into many pieces. The cut pieces are bundled together and fused to form a multicore preform. Other techniques include precision hole drilling a large glass rod and filling the holes with rare-earth doped cores. However, as the number of desired cores increases, such methods become very labor-intensive and are difficult to apply. Because these earlier arrays were produced by abrasive methods, the introduction of micro-fractures in the core region is also a distinct possibility.
Accordingly, there is a need for forming a multicore high-power fiber laser array in an economical and non-abrasive manner, such that the array features a substantially defect-free, isometric multicore structure. Moreover, the cores of the fiber should be disposed at a relatively equal distance from one another, and should be configured in a manner to produce a phase-locked output beam in favor of the fundamental in-phase supermode emerging as a high-brightness beam from the multicore fiber laser array.